As substrates for power semiconductors and short wavelength lasers, wide bandgap semiconductors have been receiving attention in recent years. In particular, silicon carbide SiC (2.9 eV to 3.0 eV), gallium nitride GaN (3.4 eV), zinc oxide ZnO (3.37 eV), diamond (5.47 eV), aluminum nitride AlN (6.0 eV) and the like, are materials particularly attracting attention, because of their high bandgaps.
However, while GaN is deposited by heteroepitaxial growth on single crystalline sapphire or single crystalline SiC as an example, the deposited GaN has a lot of defects due to a difference in lattice constant, and it is now difficult to apply the GaN to power semiconductor, high-performance lasers, etc.
Among methods for producing these single crystals, a method for producing the best quality crystal is a crystal growth method such as a hydrothermal synthesis method, and a piece (wafer) cut out of bulk crystals produced in accordance with this method is known to have the best quality. However, the crystal growth takes time, and the crystal is therefore priced extremely high, and now has failed to have a wide range of application. However, a section for actual use as a device is a limited region of several hundreds nm to several μm from the surficial layer so that a method of transferring the bulk crystals thinly onto a handle substrate to reduce cost can be said to be a natural idea.
As a typical method for film transfer, a SOITEC method can be cited, and in this method, a semiconductor film is transferred onto a handle substrate in such a way that a semiconductor substrate (donor substrate) subjected to hydrogen ion implantation in advance at room temperature and a substrate (handle substrate) to serve as a supporting substrate are bonded to each other, and subjected to a heat treatment at a high temperature (around 500° C.) to generate a large number of minute air bubbles referred to as microcavities and carry out detachment.
However, in practice, due to a difference in coefficient of thermal expansion between the donor substrate and the handle substrate (for example, silicon, quartz, sapphire, etc.), it is to be anticipated that simply bonding the both substrates and raising the temperature will cause the substrates to be cracked, thereby failing to form the composite substrate.
As another method, there is a method referred to as a SiGen method, which refers to a method in which a surface of a semiconductor substrate likewise subjected in advance to hydrogen ion implantation and/or a surface of a handle substrate is subjected to a plasma treatment to activate the surface or surfaces, and then bonded to each other, and a mechanical impact is applied to the substrates to carry out detachment at the hydrogen ion-implanted interface. However, the film transfer relies on a mechanical method (an impact or the like) so that there is a problem that the substrate is destroyed during the transfer of film from the thin semiconductor substrate with a small diameter due to the insufficient mechanical strength of the substrate.